Meta-analysis

A meta-analysis is a statistical technique that combines the result of several studies in order to test a research hypothesis. Using the technique it may be possible to reach statistically significant conclusions, even where the results of the original underlying studies are themselves not statistically significant on their own.The statistical analysis of a large collection of analysis. One of the primary purposes of "meta-analysis is to provide the same methodological rigor to a literature review that we require from experimental research" plus to "integrate findings".[1]

There are possible weaknesses to this approach:

It is harder to control sources of bias since multiple studies are involved.

A bias towards a predetermined outcome can influence the choice of studies to combine.